1. Field of the Invention
The present invention relates, in general, to methods and apparatus for locating the distance and direction to a conductive target, such as a cased well or borehole, from a remote location such as a rescue borehole or well to obtain data for use in guiding the direction of drilling the rescue well to intersect the target, and to methods and apparatus for injecting time-varying electrical currents into the earth from one or more electrodes in the rescue borehole, for detecting at the drill bit of the rescue well electromagnetic field vectors resulting from such injected currents flowing in the target, and for transmitting data representing the detected fields to the earth's surface. More particularly, the invention relates to a method and apparatus for guiding the drilling of a borehole when the rescue well is traveling in a direction very close to vertical and the direction of gravity almost coincides with the direction of drilling.
2. Description of the Related Art
It is well known that in drilling boreholes in the earth, such as deep wells for oil and gas exploration, precise control of the path followed by the well is extremely difficult, so that it is virtually impossible to know the exact location of the well at a given depth. For example, a drilling tolerance of plus or minus one quarter of a degree will allow the bottom of a 10,000-foot well to be positioned anywhere within a circle 100 feet in diameter, and numerous factors can increase the deviation. This is not of particular concern in many drilling operations, but if drilling precision is necessary, as where a borehole is to be drilled precisely to a target location, such variations can cause severe difficulties. One example of the need for precision drilling occurs in the situation where it becomes necessary to drill a relief well to intersect an existing deep well, as in the case where the casing of the deep well has ruptured and it becomes necessary to plug the well at or below the point of the rupture to bring it under control. In order to do this, the relief well must be drilled to intersect the original well at the desired level, and since such ruptures, or blowouts, often produce extremely hazardous conditions at the surface in the vicinity of the original well, the relief well usually must be started a considerable distance away from the original wellhead and drilled at an incline down to the desired point of intersection.
Because the same problems of control of the direction of drilling that were encountered in the original well are also encountered in drilling the relief well, the location of the relief well borehole also cannot be known with precision; accordingly, it is extremely difficult to determine the distance and direction from the end of the relief well to the desired point of intersection on the target well. In addition, the relief well usually is very complex, compounding the problem of knowing exactly where it is located with respect to a target that may be 10 inches in diameter at a distance of thousands of feet below the earth's surface.
Numerous early attempts were made to solve the problem of guiding a relief well to accurately intersect a target well. Some utilized surveying techniques to locate the relief well with respect to a target well, but such survey techniques are not capable of providing accurate data concerning the relationship of the relief well to the original well until the relief well has approached very near the original well. Magnetic gradient ranging equipment can be used with considerable accuracy at close range; however, it has been found that outside a radius of a few tens of feet, such systems are usually inadequate.
In an attempt to extend the distance at which accurate information can be obtained, a variety of electrical well logging techniques have been used which treat the target well as an anomaly in the geologic structure of the earth surrounding the relief well. Some of these systems are directed to the measurement of the apparent resistivity of the earth across a pair of electrodes but, since no directionality is given by this method, it is ineffective for directing a relief well toward an existing well.
In addition, there have been attempts to obtain similar data through the use of electromagnetic prospecting, where induction sensing coils mounted at right angles to each other are used in conjunction with other conventional well logging systems to determine the probable location of a target. However, such systems do not suggest the possibility of locating relatively small targets such as well bores.
Other systems have been developed for directing a second well with respect to a first well by the use of sonic detectors responsive to the sound produced by fluids flowing out of a blown well formation. However, such systems will not operate when there is no sound emanating from the target well, and, in addition, do not provide the required degree of directional and distance accuracy. Another proposal in the prior art is the use of a signal transmitter in one well and a signal receiver in the other well, wherein sound waves or magnetic fields may be used as the signals. In these latter systems, however, the target well must be accessible so that the signal source can be placed in one well and the receiver in the other, and they are not effective where the target well is not open.
Many of the difficulties outlined above were overcome in the prior art by methods and apparatus disclosed, for example, in U.S. Pat. Nos. 4,323,848, 4,372,398, 4,700,142, and 5,512,830, all issued to Arthur F. Kuckes, the applicant herein. In accordance with such prior art patents, an electric current flow is produced in a target such as the casing of a target well by injecting a low frequency alternating current into the earth surrounding the target well through the use of an electrode located in the relief well, or borehole. This current flow extends between the downhole electrode and a second electrode that may be located at the earth's surface in the vicinity of the head of the relief well. The injected earth current finds a path of least resistance through the casing or other current-conducting material in the target borehole, and the resulting concentration of current produces a characteristic magnetic field surrounding the target well which can be detected by an AC magnetic field sensor such as that described in U.S. Pat. No. 4,323,848, or by multiple sensors, as described in U.S. Pat. No. 5,512,830. These sensors are extremely sensitive to very small magnetic fields, and accurately detect the vectors of magnetic fields produced by currents flowing in well casings located a considerable distance away from the relief borehole.
The vector signals obtained from the AC magnetic field sensors, in accordance with the aforesaid patents, permit calculation of the direction and distance to the target well casing with respect to the location of the AC magnetic field sensor in the relief well. This information can be used to guide further drilling of the relief well. Thus, as the relief well approaches a desired depth, its approach to the location of the target well can be guided so that the target well is intersected at the desired depth below the earth's surface in a rapid and effective manner. This method of guiding a relief well to intersect with a target is a homing-in process, wherein multiple measurements—often after every 50 feet of drilling—must be made as the relief borehole approaches the target, so that more time is spent measuring than is spent drilling. This need for making so many measurements makes the drilling of a relief well very expensive, especially in off-shore drilling, wherein, using the prior methods, the drill string for the relief well must be pulled for each measurement.
The foregoing systems are widely, and successfully, used; however, each of them requires a periodic withdrawal of the drill string so that suitable sensors and electrodes for generating the ground current can be lowered into place and so that distance and direction measurements from the relief well to the target can be obtained. Since a drilling rig operation can cost upwards of $500,000.00 per day in offshore drilling operations, the time-consuming process of halting the drilling, withdrawing the drill string, and positioning the measuring equipment is an extremely expensive procedure Accordingly, a method and apparatus for making such measurements without the effort and expense of pulling the drill string is needed.
Furthermore, in a typical borehole drilling operation, the path of the borehole, which may be a relief well as described above, is tracked during drilling by a “measurement while drilling” (MWD) instrument that is mounted near the bottom of the drill string. Usually, such a string consists of a series of steel tubes, each about 10 meters in length and connected end-to-end. Connected at the bottom end of the drill string is a non-magnetic section which carries the MWD instrument, and below that, a hydraulic drilling motor having a bent housing to which the drill bit is connected via a drill shaft, with each of the non-magnetic section and the bent housing being about 10 meters in length. As a result of this, the MWD instrument is typically located 10-20 meters above the face of the drill bit, so that when magnetic field measurements are made with the drill string in the relief well, they are actually made a considerable distance from the drill bit, introducing a significant error in determination of the relative distance and direction of the target with respect to the drill bit. This greatly increases the difficulty of accurately controlling the intersection of the borehole being drilled with the target.
Accordingly, there was a need for a measurement system that will significantly increase the accuracy of distance and direction calculations in drilling, while reducing the cost of making such calculations.
Prior U.S. Patent Application Publication No. U.S.2010/0155138A1, referenced above, is directed to an improved method and apparatus for determining the distance and direction from the drill bit of a relief well drill string to a target location, such as the center of an existing borehole casing, without the need to withdraw the drill string to make the necessary measurements, while still making the measurements from the bottom of the relief well so that accurate calculations can be made. In accordance with one aspect of that invention, the need for pulling a drill string in order to make magnetic field measurements in a relief well, or borehole, is obviated by the use of magnetic field sensors mounted in a drill bit instrument package that is secured to the drill bit, in combination with a drill string wireline having a suitable current-injecting electrode and a wireline instrument package which can be dropped down through the center of the drill string whenever a measurement is to be made. The electrode is energized with a time-varying current to produce a corresponding magnetic field generated by current flow in the target, and the drill bit instrument detects that magnetic field at the drill bit. The drill bit instrument transmits data representing the measured field vectors, and the wireline instrument package receives that data and transmits it to the surface for use in guiding further drilling. The wireline is then withdrawn, and drilling can be resumed.
The foregoing process is carried out, in accordance with another aspect of that invention, by a modified drill string structure having at least one insulating segment, but preferably two such segments, spaced apart to electrically isolate a selected conventional tubular, electrically conductive, steel drill string pipe section near the bottom of the string to form a drill string electrode. These pipes are generally about ten meters in length and are joined end-to-end, with sections being added to the drill string as drilling progresses. Each insulating segment, or sub, is about one meter in length, so that a single sub is generally sufficient for electrical isolation, although additional subs may be used, as needed. The drill string preferably includes a single such electrode section, although in some circumstances it may be desirable to include two spaced electrode sections separated and isolated from each other by at least one insulating sub. If desired, they may be spaced further apart by including one or more non-electrode steel pipe sections between the insulating subs for the electrode sections. The modified drill string includes a nonmagnetic segment, in which is mounted a conventional MWD instrument, and the lowermost (distal) end of the drill string is a standard rotating drill bit connected to the shaft of a standard hydraulic drilling motor incorporating, in a preferred form of the invention, a bent housing for directional drilling control, in known manner. As is known, the drilling motor may be driven by drilling fluid that flows down the center of the drill string and back up the borehole outside the string.
When a magnetic field measurement is to be made using the drill string of the invention disclosed in the '138 publication, drilling is halted, and instead of withdrawing the drill string, a wireline carrying a wireline electrode is lowered through the center of the drill string until the wireline electrode is aligned with the approximate center of the corresponding isolated steel drill pipe electrode section. The wireline electrode is in electrical communication with its corresponding isolated steel drill pipe electrode section which is, in turn, in electrical communication with the surrounding earth formations. When the wireline is energized, the drill pipe electrode injects current from the wireline electrode into the surrounding formations and a portion of that current is then collected in the target. The electrodes are energized by a periodic time-varying current, such as a sinusoidal AC supplied from a power supply at the earth's surface, to produce a characteristic target current and corresponding target magnetic field. The wireline electrode is immersed in the drilling fluid, which may be electrically conductive to provide electrical communication between it and its corresponding drill pipe electrode. In the case where a non-conductive drilling fluid is used, spring-loaded contacts may be employed on the wireline electrode to provide a positive electrical contact with the inner surface of the isolated steel drill pipe section.
In accordance with the '138 publication, the desired magnetic field measurements are made at the drill bit sensor, or magnetic field detector, that is located in the drill bit instrument package described above. This location for the drill bit sensor is advantageous, because it is close to the actual location of the drill bit that is to be controlled. The drill bit instrument is battery-operated, and in addition to suitable magnetic field vector detectors and gravity vector detectors, it incorporates suitable electromagnetic telemetry, such as an electromagnetic solenoid, for transmitting data from the drill bit sensor instrument to the wireline instrument in the drill string. The wireline instrument includes suitable telemetry to remotely receive the data from the drill bit sensor and to transmit that data to the surface.
In another embodiment of the invention described in the aforementioned '138 publication, magnetic field measurement accuracy may be improved in some circumstances by operating the system in a pulsed transient mode, wherein the earth formations surrounding the relief and the target wells are energized by a stepped, or pulsed, primary excitation current from a power source which preferably is at the surface, and measurements of magnetic fields produced by the resulting current flow in the target are made immediately following a stepwise turn-off of the excitation current, when that current is zero. Each pulse of electrical energy supplied to the wireline electrode causes a current to flow through the earth's formations to the target, and, as described in the foregoing U.S. Pat. No. 4,700,142, this current is collected on the electrically conductive target. The resulting target current flow creates a characteristic target magnetic field that is detected by the drill bit sensor instrument. In the pulsed, or transient, mode of operation of the device, the magnetic field measurement is made after the primary energizing current stops. The magnetic fields that are measured when the excitation current is zero are caused by a decaying target well current flow. Although this decay current produces only a very small field, since even the primary target current typically is only a few percent of the energizing current, the measurement of the decay field is more accurate, since interfering fields caused by the primary electrode current in the earth are not present.
To enhance this transient pulsed current magnetic field measurement, the drill string incorporates at least two spaced, electrically isolated conductive drill string pipe sections, each separated from each other and other adjoining pipe sections by one or more electrically insulating subs. Deep well measurements are made by aligning corresponding spaced-apart wireline electrodes with the approximate centers of corresponding isolated drill pipe sections to effectively produce two drill pipe injection electrodes spaced along the drill string above the drill motor, by supplying a time-variable current to the electrodes to inject a current in the earth and producing a corresponding time-varying target current, and by detecting the resulting target magnetic field vectors at the location of a drill bit sub. Telemetry at the drill bit sub transmits the detected vector data uphole for use in calculating the distance and direction from the drill bit sub to the target.
The invention disclosed in the referenced '138 publication has proven to be very important for the drilling guidance of relief wells to intersect and to stop the uncontrolled flow of oil in a blowout well. As described above, a crucial element of that invention is to determine the direction to a “blowout” oil well from the relief well being drilled to enable proper adjustments to the direction of drilling, and this is done by orienting the electromagnetic instruments relative to the borehole using accelerometers to define the orientation of the plane defined by the direction of drilling and the direction of gravity, i.e., the vertical axis. However, when the relief well is very close to vertical and the direction of gravity almost coincides with the direction of drilling this method for tool orientation fails.